Process for the regeneration of zeolite catalysts

ABSTRACT

A process for the regeneration of a zeolitic catalyst which is at least partially exhausted by use in the synthesis of optionally substituted methylenedianiline (MDA) and derivatives thereof, or of a mixture of optionally substituted MDA and derivatives thereof, with a higher homologous product the process involving washing said catalyst with an aromatic compound having at least one substitutent on the aromatic ring having activating characteristics with respect to the electrophilic substitution, in liquid or at least partially liquid phase.

The present invention relates to a process for the regeneration of azeolitic catalyst. More specifically, the present invention relates to aprocess for the regeneration of a zeolitic catalyst used in thepreparation of optionally substituted methylenedianiline (MDA) andderivatives thereof, or of mixtures of optionally substituted MDA andderivatives thereof, and a higher homologous product. Suitably,optionally substituted MDA and derivatives thereof and mixtures ofoptionally substituted MDA and derivatives thereof together with ahigher homologous product contain a compound of formula (I):

wherein R represents independently hydrogen, a C₁–C₈ alkyl, C₄–C₁₀cycloalkyl C₆–C₁₂ aromatic radical and n is an integer greater than orequal to one, such as to give a functionality ranging from 2 to 6. n issuitably from 1 to 5.

The term “derivatives thereof” when used in relation to MDI denotescompounds in which one or more of the amine groups attached to thearomatic rings is a secondary amine, that is where R is not hydrogen.

The term higher “homologous product” denotes a compound of formula Iwhere n is at least 2 and thus comprises a compound having at least 3aromatic rings each with an amine group and being optionally substitutedand being linked by methylene groups.

Optionally substituted methylenedianiline and derivatives thereof ormixtures of optionally substituted methylenedianiline and derivativesthereof, with a higher homologous product, may be used as intermediatesin the preparation of isocyanates in which at least one of the aminegroups is converted into an isocyanate group, hereinafter referred to asa “corresponding isocyanate”. The corresponding isocyanates may be usedin the synthesis of different types of compounds including for examplepolyurethanes, thermoplastic polymers and epoxy resins.

Methylenedianiline may be produced from aniline or one of itsderivatives by condensation with formaldehyde in the presence of astrong acid solution, for example hydrochloric acid, sulfuric acid andphosphoric acid, as described, for example, in U.S. Pat. Nos. 2,683,730,3,277,173, 3,344,162, 3,362,979 or in H. Ulrich, “Chemistry andTechnology of Isocyanates” John Wiley and Sons, USA, 1996. The operatingconditions necessary for producing a product with certain structuralcharacteristics and without the formation of significant quantities ofby-products, may require the use of a large quantity of a strong acid.Under these circumstances, materials capable of resisting these acidsare suitably used in the plant. Such materials are often expensive.Furthermore, once MDA has been synthesized, a corresponding quantity ofbase (typically sodium carbonate) is typically used to neutralize theacid used, causing the formation of large quantities of salts which mustbe disposed of. All these requirements lead to an increase in theproduction costs and difficulties in running the process.

In order to address the above drawbacks, improvements in processes forproducing MDI have been proposed, which have led to the use of solidacid catalysts in substitution of the traditional inorganic acids. U.S.Pat. Nos. 4,039,580, 4,039,581, 4,092,343 and 4,294,987, for example,describe the use of clays and diatomaceous earth as catalysts in thesynthesis of MDA. U.S. Pat. No. 5,241,119 describes a process for thepreparation of 4,4′-diamino-diphenyl methane which comprises thereaction between aniline and formaldehyde in the presence of a solidcatalyst selected from zeolites, in particular Y zeolite, ZSM-5 zeolite,zeolites modified with one or more of the following metals: aluminum,boron, iron and titanium. The reaction is carried out in a solvent at atemperature ranging from 50 to 200° C., at a pressure depending on theboiling point of the solvent used.

Italian patent applications M199A-1171 and M199A-1988 M12000A681describe synthesis processes of MDA and its higher homologous products,respectively using, as solid acid catalysts, zeolites with a“spaciousness index” ranging from 2.5 to 19, for example beta zeolite,or silico-aluminas amorphous to X-rays, with a molar ratio SiO₂/Al₂O₃from 10/1 to 500/1 and having a surface area from 500 to 1000 m²/g, aporosity from 0.3 to 0.6 ml/g and a pore diameter from 20 to 500 Å.

Solid catalysts, however, may also have disadvantages as they may losetheir activity with use due to the formation of pitches and/orcarbonaceous residues. It is believed that the residues are deposited onthe solid catalyst, block the pores in the solid and therefore reduceits contact surface. For this reason, solid acid catalysts are typicallyregenerated at a high temperature, even higher than 500° C., and in thepresence of an oxidizing gas, air or oxygen, with a consequent increasein the production costs.

The Applicants have now found a process for the regeneration of azeolitic catalyst which is at least partially exhausted by use in thesynthesis of optionally substituted methylenedianiline (MDA) andderivatives thereof or of mixtures of optionally substituted MDA andderivatives thereof with a higher homologous product starting from there-arrangement reaction of the corresponding aminal intermediate or thedirect condensation reaction between formaldehyde and optionallysubstituted aniline or a derivative thereof, the process comprisingsubjecting said catalyst to a treatment of the chemical type thusavoiding, postponing or reducing the need for high temperature thermaltreatment.

The object of the present invention therefore relates to a process forthe regeneration of a zeolitic catalyst which is at least partiallyexhausted by use in the synthesis of optionally substitutedmethylenedianiline (MDA) and derivatives thereof or of mixtures ofoptionally substituted MDA and derivatives thereof with a higherhomologous product, which comprises contacting, preferably washing, saidcatalyst with an aromatic compound, in at least partially liquid phasewherein the aromatic compound comprises a substituent having activatingcharacteristics with respect to electrophilic substitution as comparedto the aromatic compound without the said substituent.

The term “at least partially exhausted” in relation to the catalyst tobe regenerated denotes a reduction in the activity and/or selectivity ofthe catalyst which has been used in a process as compared to theactivity and/or selectivity of the catalyst prior to its use in theprocess.

By “corresponding aminal intermediate” is meant the amine-containingcompound which is capable of undergoing a rearrangement reaction toproduce the desired product, optionally substituted MDI and derivativesthereof.

Suitably, the optionally substituted MDI and derivatives thereof and, ifpresent, a higher homologous product are produced by a processcomprising the re-arrangement reaction of the corresponding aminalintermediate or the condensation reaction, preferably directcondensation reaction, between formaldehyde and optionally substitutedaniline or a derivative thereof.

Preferably the condensation or rearrangement reaction is carried out atelevated temperature.

Preferably the catalyst is contacted with the aromatic compound at atemperature higher than that of the condensation and/or re-arrangementreaction in which the optionally substituted MDI and derivatives thereofis produced.

Preferably the catalyst is contacted, especially washed with thearomatic compound in equicurrent or concurrent or countercurrent flowwith respect to the flow of the process reactants used to make theproduct.

In a preferred embodiment of the invention, there is provided a processfor the regeneration of a zeolitic catalyst which is at least partiallyexhausted by use in the synthesis of optionally substitutedmethylenedianiline (MDA) and derivatives thereof or of mixtures ofoptionally substituted MDA and derivatives thereof with a higherhomologous product by a process comprising the re-arrangement reactionof the corresponding aminal intermediate or the condensation reactionbetween formaldehyde and optionally substituted aniline or a derivativethereof, which regeneration process comprises contacting, preferablywashing, said catalyst with an aromatic compound, in at least partiallyliquid phase and at a temperature higher than that of the condensationand/or re-arrangement reaction in which the optionally substituted MDIand derivatives thereof is produced wherein the aromatic compoundcomprises a substituent having activating characteristics with respectto electrophilic substitution as compared to the aromatic compoundwithout the said substituent.

Suitably, the regeneration process may be repeated to successivelyregenerate the catalyst as desired.

The invention further provides a process for producing optionallysubstituted methylenedianiline (MDA) and derivatives thereof or ofmixtures of optionally substituted MDA and derivatives thereof with ahigher homologous product by a process comprising:

A) contacting a feedstock comprising i) the corresponding aminalintermediate or ii) formaldehyde and optionally substituted aniline or aderivative thereof with a zeolitic catalyst at elevated temperature toeffect, in the case of feedstock i), a rearrangement reaction and, inthe case of feedstock ii) a condensation reaction, to produce optionallysubstituted methylenedianiline (MDA) or a derivative thereof;

B) regenerating the at least partially exhausted catalyst by a processcomprising contacting said catalyst with an aromatic compound, in atleast partially liquid phase wherein the aromatic compound comprises asubstituent having activating characteristics with respect toelectrophilic substitution as compared to the aromatic compound withoutthe said substituent at a temperature higher than that of step A); andoptionally

C) alternately further carrying out step A) and step B).

Preferably, the process described immediately above comprises a seriesof process steps of producing the desired product as described in stepA), regenerating the catalyst as described in step B) and optionallyrepeating those steps in succession.

The aromatic compound for use in the present process for regeneratingthe catalyst comprises at least one substituent of the ring having anactivating effect with respect to electrophilic substitution which isstrong or intermediate as compared to the compound without the saidsubstituent. Suitable substituents having a strong activating effectinclude amines, for example —NH₂, —NHR, and —NR₂, wherein R is asdefined in formula I and —OH, and —O⁻. Suitable substituents having anintermediate activating effect include for example, —OR, —NHCOR₂substituents wherein R₁ and R₂ represent a C₁–C₄ (iso) alkyl, C₄–C₁₀cycloalkyl, aromatic, alkylammatic or C₆–C₁₀ arylalkyl radical. Othersubstituents which provide an activating effect with respect toelectrophilic substitution may also be employed as desired.

Preferred aromatic compounds according to the present invention arephenol and aromatic amines, especially aniline.

Any solid acid catalyst suitable for use in a re-arrangement and/orcondensation reaction to produce optionally substituted MDA andderivatives thereof, and/or optionally substituted MDA and derivativesthereof mixed with a higher homologous product, may be subjected to thepresent regeneration process.

Catalysts particularly suitable for regeneration in the present processinclude zeolites having a “spaciousness index” from 2.5 to 19,silico-aluminas amorphous to X-rays, with a molar ratio SiO₂/Al₂O₃ from10/1 to 500/1 and having a surface area from 500 to 1000 m²/g, aporosity from 0.3 to 0.6 ml/g and a pore diameter from 2 to 50 nm,described in the above-mentioned Italian patent applications, togetherwith the description of the synthesis of optionally substituted MDA, oroptionally substituted MDA, mixed with a homologous higher product.

These catalysts may be used in any suitable form including, in the formof compressed powders, or extruded bodies (spheres, pellets or tablets)suitably after blending with an extrusion ligand.

The “spaciousness index” is a parameter which provides the realmeasurement of the pore amplitude of porous materials such as zeolites,of which a detailed description can be found in literature, for examplein “Zeolites and Related Microporous Material: State of the Art 1994”,Studies in Surface Science and Catalysis, vol. 84, 37, 1994, ElsevierScience B.V.

Preferably the at least partially exhausted catalyst is contacted,especially washed with the aromatic compound at a higher temperaturethan that of the re-arrangement and/or condensation reaction by whichthe optionally substituted MDI and derivatives thereof or optionallysubstituted MDA and derivatives thereof mixed with a homologous higherproduct are preferably produced.

As the rearrangement or condensation reaction is suitably carried out ata temperature from 50 to 200° C., the regeneration process is preferablyeffected at a temperature from 100 to 400° C., more preferably from 200to 320° C. The pressure of the container where the regeneration iscarried out is such as to maintain the aromatic compound in liquid orpartially liquid phase.

Some illustrative but non-limiting examples are provided for a betterunderstanding of the present invention and for its embodiment.

EXAMPLE 1 Synthesis of Aminal (Reaction Intermediate)

The reaction intermediate having the general formula:

is prepared by condensation between aniline and formaldehyde. Inparticular, an aqueous solution at 37% of formaldehyde is charged, understirring, into a reaction container containing aniline, so that themolar ratio of formaldehyde to aniline is four. The temperature isslowly increased to 50° C.

At the end of the addition the mixture is stirred for an hour and theorganic phase consisting of aminal and non-reacted aniline is thenseparated in a separator funnel. The organic phase is then dried to amaximum water content of 1.25% and conserved for subsequent use.

EXAMPLE 2 Aminal at 30%—Regeneration with Aniline

10 cm³ of beta zeolite extruded with a quantity of ligand (Al₂O₃) equalto 50% by weight, sieved to 70–100 mesh, was charged into a tubularreactor having a diameter of 12.5 mm and a length of 390 mm. A mixtureof aminal (prepared according to Example 1) at 30% by volume in anilinewas then fed to the reactor, at a temperature of 180° C., a pressure of4 bars and an LHSV of 7.2 h⁻¹.

A total conversion of the-aminal feed is obtained, with a concentrationof methylenedianiline (4,4′ MDA+2,4′ MDA) in the reaction product of90%, the complement to 100% being trimers and tetramers of MDA. The testwas carried out until the concentration of MDA in the reaction mixturedropped to below 84%. At this point the regeneration procedure of thepartially exhausted catalyst was carried out.

The feeding of aminal was interrupted, and pure aniline was fed at thesame space velocity. The catalytic bed was heated to a temperature of230° C. creating a counter-pressure in the reactor of six bars and thetemperature reached was maintained for 12 hours.

The catalytic bed was then brought back to the reaction temperature(180° C.) and, at the end of the reaction procedure described, thefeeding of aminal was re-started, obtaining a total conversion of theaminal with a concentration of methylenedianiline (4,4′ MDA+2,4′ MDA) inthe reaction product of 90.5%, the complement to 100% being trimers andtetramers of MDA.

The test was prolonged until the concentration of MDA in the reactionmixture dropped to below 81%. At this point a regeneration procedure ofthe partially exhausted catalyst, as described above, was carried out.

At the end of this second regeneration, the feeding of aminal wasre-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 90.1%, the complement to 100% being trimers and tetramers ofMDA.

The test was carried out until the concentration of MDA in the reactionmixture dropped to 37%; in this case the conversion of aminal was nolonger total but decreased to 71%. At this point a regenerationprocedure of the partially exhausted catalyst as described above, wascarried out.

At the end of this third regeneration, the feeding of aminal wasre-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 90.25%, the complement to 100% being trimers and tetramers ofMDA.

EXAMPLE 3 Aminal at 70%—Regeneration with Aniline

10 cm³ of beta zeolite extruded with a quantity of ligand (Al₂O₃) equalto 50% by weight, sieved to 70–100 mesh, was charged into a tubularreactor having a diameter of 12.5 mm and a length of 390 mm. A mixtureof aminal (prepared according to Example 1) at 70% by volume in anilinewas then fed to the reactor, at a temperature of 180° C., a pressure of4 bars and an LHSV of 7.2 h⁻¹.

A total conversion of the aminal feed was obtained, with a concentrationof methylenedianiline (4,4′ MDA+2,4′ MDA) in the reaction product of72%, the complement to 100% being trimers and tetramers of MDA.

The test was continued until the concentration of MDA in the reactionmixture dropped to 47%, in this case there being no longer totalconversion of the aminal which decreased to 94%. At this point theregeneration procedure of the partially exhausted catalyst wasactivated. The feeding of aminal was interrupted, and pure aniline fedat the same space velocity. The catalytic bed was heated to atemperature of 250° C. creating a counter-pressure in the reactor of sixbars and the temperature reached was maintained for 12 hours.

The catalytic bed was then brought back to the reaction temperature(180° C.).

At the end of the reaction procedure described, the feeding of aminalwas re-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 72.7%, the complement to 100% being trimers and tetramers ofMDA.

EXAMPLE 4 Pure Aminal—Regeneration with Aniline)

10 cm³ of beta zeolite extruded with a quantity of ligand (Al₂O₃) equalto 50% by weight, sieved to 70–100 mesh, was charged into a tubularreactor having a diameter of 12.5 mm and a length of 390 mm. Pure aminal(prepared according to Example 1) was then fed to the reactor, at atemperature of 180° C., a pressure of 4 bars and an LHSV of 7.2 h⁻¹.

A total conversion of the aminal feed was obtained, with a concentrationof methylenedianiline (4,4′ MDA+2,4′ MDA) in the reaction product of66%, the complement to 100% being trimers and tetramers of MDA. The testwas continued until the concentration of MDA in the reaction mixturedropped to below 43%. At this point the regeneration procedure of thepartially exhausted catalyst was activated.

The feeding of aminal was interrupted, pure aniline was fed at the samespace velocity and the catalytic bed was heated to a temperature of 250°C. creating a counter-pressure in the reactor of six bars.

The temperature reached was maintained for 12 hours, and the catalyticbed was then brought back to the reaction temperature (180° C.).

At the end of the reaction procedure described, the feeding of pureaminal was re-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 66.5%, the complement to 100% being trimers and tetramers ofMDA.

EXAMPLE 5 Comparative

10 cm³ of beta zeolite extruded with a quantity of ligand (Al₂O₃) equalto 50% by weight, sieved to 70–100 mesh, was charged into a tubularreactor having a diameter of 12.5 mm and a length of 390 mm. A mixtureof aminal (prepared according to the Example 1) at 30% by volume inaniline was then fed to the reactor, at a temperature of 180° C., apressure of 4 bars and an LHSV of 7.2 h⁻¹.

A total conversion of the aminal feed was obtained, with a concentrationof methylenedianiline (4,4′ MDA+2,4′ MDA) in the reaction product of66.5%, the complement to 100% being trimers and tetramers of MDA. Thetest was continued until the concentration of MDA in the reactionmixture dropped to below 42%. At this point the regeneration procedureof the partially exhausted catalyst was carried out.

The feeding of aminal was interrupted and mesitylene(1,3,5-trimethylbenzene) was fed at the same space velocity. Thecatalytic bed was heated to a temperature of 250° C. creating acounter-pressure in the reactor of eight bars.

The temperature reached was maintained for 12 hours, and the catalyticbed was then brought back to the reaction temperature (180° C.).

At the end of the reaction procedure described, the feeding of aminalwas re-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 37%, the complement to 100% being trimers and tetramers ofMDA. The selectivity of the

catalyst as regenerated was not as high as the initial selectivity levelof the catalyst.

EXAMPLE 6 Comparative

10 cm³ of beta zeolite extruded with a quantity of ligand (Al₂O₃) equalto 50% by weight, sieved to 70–100 mesh, was charged into a tubularreactor having a diameter of 12.5 mm and a length of 390 mm. A mixtureof aminal (prepared according to Example 1) at 30% by volume in anilinewas then fed to the reactor, at a temperature of 180° C., a pressure of4 bars and an LHSV of 7.2 h⁻¹.

A total conversion of the aminal feed was obtained, with a concentrationof methylenedianiline (4,4′ MDA+2,4′ MDA) in the reaction product of90%, the complement to 100% being trimers and tetramers of MDA. The testwas continued until the concentration of MDA in the reaction mixturedropped to below 84%. At this point the regeneration procedure of thepartially exhausted catalyst was carried out.

The feeding of aminal was interrupted, n-decane was fed at the samespace velocity and the catalytic bed was heated to a temperature of 230°C.

The temperature reached was maintained for 12 hours, and the catalyticbed was then brought back to the reaction temperature (180° C.).

At the end of the reaction procedure, the feeding of aminal at 30% wasre-started, obtaining a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 72%, the complement to 100% being trimers and tetramers ofMDA.

The test was continued for the same period of time as the test describedin Example 2 and in this case the concentration of MDA in the reactionproduct dropped to 43% whereas there was not total conversion of aminalthis being at 69%.

The regeneration was repeated with n-decane as described above and atthe end of the reaction process described, the feeding of aminal at 30%was re-started, providing a total conversion of the aminal with aconcentration of methylenedianiline (4,4′ MDA+2,4′ MDA) in the reactionproduct of 39%. The aminal conversion was no longer total but was 63%.

The test was carried out for the same period as the test described inExample 2, in this case the concentration of MDA in the reaction productdropped to 21% whereas the aminal conversion was no longer total but wasat 42%.

1. A process for the regeneration of a zeolitic catalyst which is atleast partially exhausted by use in the synthesis of optionallysudstituted methylenedianiline (MDA) and derivatives thereof or of amixture of optionally substituted MDA and derivatives thereof with ahigher homologous product, which comprises contacting said catalyst withan aromatic compound, in at least partially liquid phase wherein thearomatic compound comprises at least one substituent having activatingcharacteristics with respect to electrophilic substitution as comparedto the aromatic compound without the said substituent wherein thesubstituent having activating characteristics is selected from NH₂,—NHR, —NR₂, —OH, —O⁻, —OR¹, or —NHCOR² substituents wherein R ishydrogen, a C₁–C₈ alkyl, C₄–C₁₀ cycloalkyl C₆–C₁₂ aromatic radical. R¹and R² represent a C₁–C₄ (iso)alkyl, C₄–C₁₀ cycloalkyl, aromatic,alkylaromatic or C₆–C₁₀ arylakyl radical.
 2. A process according toclaim 1 in which the optionally substituted methylenedianiline (MDA) andderivatives thereof or the mixture of optionally substituted MDA andderivatives thereof with a higher homologous product comprise a compoundof formula (I):

wherein R represents independently hydrogen, a C₁–C₈ alkyl, C₄–C₁₀cycloalkyl C₆–C₁₂ aromatic radical and n is an integer greater than orequal to one, such as to give a functionality ranging from 2 to
 6. 3. Aprocess according to any one of claims 1 and 2 wherein the at least onesubstituent of the aromatic ring has a strong activating effect and isselected from —NH₂, —NHR, —NR₂, —OH or —O⁻ groups.
 4. A processaccording to any one of claims 1 and 2 wherein the at least onesubstituent of the aromatic ring has an intermediate activating effectand is selected from —OR¹, or —NHCOR² groups wherein R¹ and R² representa C₁–C₄ (iso) alkyl, C₄–C₁₀ cycloalkyl, aromatic, alkylaromatic orC₆–C₁₀ arylalkyl radical.
 5. A process according to claim 1 wherein thearomatic compound is selected from phenol and aniline.
 6. A processaccording to claim 1 wherein the catalyst is selected from zeolites witha “spaciousness” index ranging from 2.5 to 19, or silico-aluminasamorphous to X-rays, with a molar ratio SiO₂/AI₂O₃ from 10/1 to 500/1and having a surface area from 500 to 1000 m²/g, a porosity from 0.3 to0.6 ml/g and a pore diameter from 2 to 50 nm.
 7. A process according toclaim 1 in which the optionally substituted MDA and derivatives thereofand, if present, a higher homologous product are produced by a processcomprising the re-arrangement reaction of the corresponding aminalintermediate of the condensation reaction between formaldehyde andoptionally substituted aniline or a derivative thereof at elevatedtemperature.
 8. A process according to claim 7 in which the catalyst iscontacted with the aromatic compound at a temperature higher than thatof the condensation and/or re-arrangement reaction in which theoptionally substituted MDA and derivatives thereof is produced.
 9. Aprocess according to any one of claims 7 and 8 in which the catalyst iscontacted, with the aromatic compound in equicurrent or concurrent orcountercurrent flow with respect to the flow of the process reactantsused to make the product.
 10. A process according to claim 1 wherein thecontacting of the catalyst with the aromatic compound takes place at atemperature ranging from 100 to 400° C.
 11. A process for theregeneration of a zeolitic catalyst which is at least partiallyexhausted by use in the synthesis of optionally substitutedmethylenedianiline (MDA) and derivatives thereof or of mixtures ofoptionally substituted MDA and derivatives thereof with a higherhomologous product by a process comprising the re-arrangement reactionof the corresponding aminal intermediate of the condensation reactionbetween formaldehyde and optionally substituted aniline or a derivativethereof, which regeneration process comprises contacting, said catalystwith an aromatic compound, in at least partially liquid phase and at atemperature higher than that of the condensation and/or re-arrangementreaction in which the optionally substituted MDA and derivatives thereofis produced wherein the aromatic compound comprises a substituent havingactivating characteristics with respect to electrophilic substitution ascompared to the aromatic compound without the said substituent whereinthe substituent having activating characteristics is selected from NH₂,—NHR, —NR₂, —OH, —O⁻, —OR¹, or —NHCOR² substituents wherein R ishydrogen, a C₁–C₈ alkyl, C₄–C₁₀ cycloalkyl C₆–C₁₂ aromatic radical, R¹and R² represent a C₁–C₄ (iso)alkyl, C₄–C₁₀ cycloalkyl, aromatic,alkylaromatic or C₆–C₁₀ arylakyl radical.